Discovery of millihertz quasi-periodic oscillations in the X-ray binary EXO 0748--676

• Published in: arXiv.org 2021-02
• Main Authors: Mancuso, G C, Altamirano, D, García, F, Lyu, M, Méndez, M, Combi, J A, Díaz-Trigo, M, J J M in't Zand
• Format: Article
• Language: English
• Subjects: Binary stars; Frequency drift; Inclination; Quasi-Periodic Oscillations; X ray binaries; X ray stars
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1905.01956

We report the discovery of millihertz quasi-periodic oscillations (mHz QPOs) from the bursting, high-inclination atoll neutron-star low-mass X-ray binary (NS LMXB) EXO 0748--676 with the Rossi X-ray Time Explorer (RXTE). This class of QPO, originally discovered in three NS LMXBs, has been interpreted as a consequence of a special mode of nuclear burning on the NS surface. Using all the RXTE archival observations of the source, we detected significant (\(>3\sigma\)) mHz QPOs in 11 observations. The frequency of the oscillations was between \(\sim\) 5 and \(\sim\) 13 mHz. We also found a decrease of the QPO frequency with time in two occasions; in one of these the oscillations disappeared with the onset of an X-ray burst, similar to what was reported in other sources. Our analysis of the X-ray colours revealed that EXO 0748--676 was in a soft spectral state when it exhibited the QPOs. This makes EXO 0748--676 the sixth source with mHz oscillations associated to marginally stable burning, and the second one that shows a systematic frequency drift. Our results suggest that the mechanism that produces the drift might always be present if the mHz QPOs are observed in the so-called intermediate state.